Provide BReferenceable in libreferenceable.a, too.

* the upcoming multibyte-implementation in libroot's ICU locale
  backend is going to use this, so it's not good enough to provide
  BReferenceable only in libbe.so

src/kits/support/Jamfile

@@ -23,3 +23,5 @@ MergeObject <libbe>support_kit.o :
 	String.cpp
 	Url.cpp
 ;
+
+StaticLibrary libreferenceable.a : : [ FGristFiles Referenceable.o ] ;